Conventional techniques for fabricating a metal lead frame include the following methods.
1) In mechanical punching technology, a metal sheet is punched by a top blade and a bottom blade from top to bottom or from bottom to top longitudinally, so that a die pad for carrying a chip, an inner lead for signal transmission and an outer lead connected to an external PCB (Printed Wiring Board) can be formed in a lead frame in the metal sheet, and afterwards a metallic electroplated coating is coated on specific regions of the inner lead(or) and the die pad, to form a useable lead frame (referring to FIGS. 76 to 78).
2) In chemical etching technology, exposing, developing, windowing and chemical etching are performed on a metal sheet, so that a die pad for carrying a chip, an inner lead for signal transmission and an outer lead connected to an external PCB can be formed in a lead frame in the metal sheet, and afterwards a metallic electroplated coating is coated on specific regions of the inner lead and(or) the die pad, to form a useable lead frame (referring to FIG. 79 to 80).
3) Based on method 1) and method 2), another method further includes pasting a high temperature adhesive film with resistance to 260 degrees Celsius on the back of the lead frame which already includes the die pad for carrying a chip, the inner lead for signal transmission, the outer lead connected to an external PCB and the metallic electroplated coating on specific regions of the inner lead and(or) the die pad, to form a lead frame which can be used in a quad no-lead package and a scale plastic package (referring to FIG. 81).
4) Based on method 1) and method 2), another method further includes prepackaging the lead frame which includes the die pad for carrying a chip, the inner lead for signal transmission, the outer lead connected to an external PCB and metallic electroplated coating on specific regions of the inner lead and the die pad, and filling punched or chemically etched regions in the metal sheet with a thermoset epoxy resin, to form a pre-filled lead frame which can be used in a quad no-lead package and a scale plastic package and a package with copper wire bonding (referring to FIG. 82).
The disadvantages of the conventional technique are described as follows.
1) Lead Frame Formed through Mechanical Punching
a. In mechanical punching, a top blade and a bottom blade punch from top to bottom or from bottom to top to form a vertical section, thus any other function or component cannot be embedded into the lead frame, for example, a system object can not be integrated into the metal lead frame.
b. in mechanical punching, an edge of a metal sheet is extruded through a top blade and a bottom blade to form a extended metallic region, and length of the extended metallic region formed through extrusion can only be 80% of thickness of the lead frame at most. In a case that the length is larger than 80% of the thickness of the lead frame, the extended metallic region formed through extrusion is prone to problems such as a warpage, a subfissure, a rupture, an irregular shape and a surface hole, and an ultrathin lead frame is more prone to these problems (referring to FIGS. 85 to 86).
c. In a case that the length of the extended metallic region formed through mechanical punching is smaller than or exactly 80% of the thickness of the lead frame, the extended length is not enough for embedding a related object in the extended metallic region, especially for an ultrathin lead frame (referring to FIGS. 85 and 86).
2) Lead Frame formed through Chemical Etching
a. In chemical etching, space for embedding a component can be obtained through partial-etching, and the major disadvantage thereof is the difficulty to control a etching depth and a flatness of an etched plane.
b. After partially-etched regions for embedding components are formed in a metal sheet, a structural strength of the lead frame becomes very small, which will directly influence operation conditions (such as, pick-and-place, transportation, a high temperature, high pressure and thermal shrinkage) required for embedding an object later.
c. The lead frame formed through chemical etching can only present a pattern of an outer lead or an inner lead on the top surface and the back of the lead frame, and cannot present a system-in-package metal lead frame with a multilayer three-dimensional metal wiring.